1. Field of the Invention
The present invention relates generally to a phototaking lens system, and more particularly to a phototaking lens system used with electronic cameras using CCD imaging elements, video cameras, monitor cameras, cameras mounted on vehicles, etc.
2. Discussion of Related Art
So far, phototaking lens systems using imaging elements such as CCDs have been required to have a good telecentric property on image sides thereof for the purpose of preventing color shading due to color filters located in front of solid-state image elements or light bundle leaks due to microlenses. The phototaking lens systems are also required to have a back focus long enough to receive optical elements such as low-pass and color filters between the final surfaces thereof and the imaging elements. For this reason, they are often designed in the form of a retrofocus type comprising two groups, one being a front group having negative power and the other a rear group having positive power. This retrofocus type is favorable for making sure of the back focus, but tends to produce large quantities of off-axis aberrations such as coma, astigmatism and distortion because the lens layout is asymmetric with respect to an aperture stop so that a light bundle diverged by the front group is converged by the rear group. For a phototaking lens system having a wide field angle, a doublet consisting of a low-dispersion convex lens and a high-dispersion concave lens is often located between an aperture stop and an image side thereof for the purpose of making good correction for chromatic aberrations. To keep the same power throughout the rear group, therefore, it is required to increase the power of the positive lens. To make good correction for the off-axis aberrations and the chromatic aberrations as well, therefore, it is required to increase the number of lenses involved. However, this is undesirable because of an increase in the cost of the lens system. Such problems may be solved by making the powers of individual optical elements weak, but another problem arises, that is, the overall length of the lens system becomes too long. Recently developed wide-angle lens systems comprising a reduced number of lenses, for instance, are disclosed in JP-A's 6-148518, 7-333499 and 8-5908. These lens systems are each composed of a small number of lenses, for instance, three or four lenses, with a rear group comprising a doublet consisting of a low-dispersion positive lens and a high-dispersion negative lens, thereby making correction for chromatic aberrations. A wide-angle lens system well corrected for off-axis aberrations and chromatic aberrations, for instance, is disclosed in JP-A 6-107070. This lens system comprises about five lenses, as described in examples of the publication.
On the other hand, JP-A's 6-230275 and 4-43607 disclose wide-angle lens systems comprising two lenses, i.e., a negative lens and a positive lens, each having a Fresnel lens surface. A Fresnel lens makes use of the refraction of light, and makes good correction for off-axis aberrations due to its aspheric effect but has little action on the correction of chromatic aberrations, i.e., longitudinal chromatic aberration and chromatic aberration of magnification.
Recently, attention has been paid to a diffractive optical element (DOE) making use of a diffraction phenomenon. Unlike a conventional refractive lens, the DOE is optically characterized by having inverse dispersion and extraordinary dispersion properties, as expressed by an Abbe number .nu.=-3.453. This has already been proposed in an article by Thomas Stone and Nicholas George, "Hybrid diffractive-refractive lenses and achromats", Applied Optics, 27, 14, 2960-2971 (1988.7.15). It is thus well known to utilize these properties to make correction for chromatic aberrations with a cemented lens consisting of a refractive lens having positive power and a diffractive optical element having weak positive power. This is more favorable than a conventional cemented lens in view of the correction of aberrations because the powers of individual lenses can be diminished due to the fact that they are positive, so that off-axis aberrations can be reduced.
In recent years, many optical systems using DOEs have been put forward. Examples of U.S. Pat. No. 5,148,314 disclose eyepieces, Petzval type lenses, large-aperture lenses, telephoto lenses, etc., and teach that if DOEs are used in these optical systems, it is then possible to make correction for chromatic aberration, and other off-axis aberrations. Further, JP-A's 6-324262, 6-331898, 6-331941 and 6-347700 filed by the applicant disclose applications of DOEs to telephoto lenses, objectives, projection lenses, etc. One application of a DOE to a wide-angle lens system is an endoscope objective disclosed in JP-A 6-194571 filed by the applicant. However, one example of this publication discloses an optical system peculiar to an endoscope objective which, albeit having a wide angle, produces considerably large distortion. Thus, this system is not preferable for the field to which the present invention belongs.
Some wide-angle lens systems comprising a reduced number of lenses are available, as disclosed in JP-A's 6-148518, 7-333499 and 8-5908. These systems are well corrected for longitudinal chromatic aberration, but cannot be for chromatic aberration of magnification and off-axis aberrations. A wide-angle lens system sufficiently corrected for longitudinal chromatic aberration and chromatic aberration of magnification, and satisfactorily corrected for off-axis aberrations as well, for instance, is disclosed in JP-A 6-107070. For this lens system, however, it is required to increase the number of lenses inclusive of a cemented lens.
In view of the aforesaid problems with conventional lens systems, it is a primary object of the present invention to provide an inexpensive phototaking lens system comprising a reduced number of lenses, which makes use of a diffractive optical element to make good correction for first-order longitudinal chromatic aberration and chromatic aberration of magnification while high image-forming capability is maintained even at peripheral areas of the field angle, and which can eliminate higher-order chromatic aberrations of magnification produced in a wide-angle lens system.